1. Field of the Invention
The present invention relates to a loop-type data transmission/reception network which can assure trouble-free transmission and reception of information from and to each data communication station if any part of the data transmission/reception route (i.e., common data bus) is disconnected.
2. Description of the Prior Art
A conventional loop-type network of data transmission/reception is exemplified by Japanese Patent Application Publication No. 57-129048, the disclosure of which is hereby incorporated by reference.
The disclosed system comprises, e.g., as shown in FIG. 1, three stations A, B, and C interconnected via a single data transmission/reception route (common data bus) to form a loop, the processed data in each station being transferred to the adjacent station only in one direction as shown by the arrow in FIG. 1, i.e., the counterclockwise direction as viewed in FIG. 1.
On the other hand, in systems connected in a loop with a single data transmission/reception route, when a break in the transmission/reception route occurs, the whole system will be hung up since no information can be transferred and, therefore, it cannot be transmitted past the station at which the data transmission/reception route has broken down. Therefore, to prevent such a system hang-up, a countermeasure (system back-up) has been commonly used-namely, a so-called loop-back method to be described below.
That is to say, if the part of the data transmission/reception route labelled L.sub.3 for unidirectionally transmitting information from the station C to the station A is broken (disconnected), the station A detects the break-down of the data transmission/reception route L.sub.3 due to the absence of received data from the station C for a predetermined period of time and informs the remaining stations B and C that leg L.sub.3 of data transmission/reception route has broken down. Thereafter information can still be sent to each station by performing bidirectional transmission between each station via the remaining legs of the data transmission/reception route at a predetermined timing in a sequence which can be illustrated as A.fwdarw.B.fwdarw.C.fwdarw.B.fwdarw.A.fwdarw.B . . . .
As described above, the loop-back processing is carried out if one leg of the data transmission/reception route breaks down.
As shown in FIG. 2, each station (station B is shown as an example in FIG. 2) is provided with a switch circuit 1 which controls the direction of transmission/reception of data. During a normal operation of the data transmission/reception ring (i.e., the data is transmitted or received in the unidirection), an input bus In is connected to a leg of the data transmission/reception route L.sub.1 leading from the station A at the previous stage of the network and an output bus OUT is connected to another leg of the transmission/reception route L.sub.2 leading to the station C at the next stage.
In such a configuration, if the break-down described above occurs, the switch positions of the switch circuit 1 are changed at predetermined intervals to reverse the directions of data transmission and reception.
In FIG. 2, numeral 2 denotes a Central Processing Unit for controlling the overall data processing performed at each station, and numeral 3 denotes an interface called SCI (Serial Communication Interface) for inputting and outputting the transmission/reception signals serving to convert the bits of information from parallel (byte-wide) form to serial (bit-wide) form and from serial to parallel form. In addition, the Read-Only Memory (ROM) 4 and Random-Access Memory (RAM) 5 are memories used by the microcomputer, and interface 6 denotes an input/output port used by the microcomputer. One example of a microcomputer of such configuration is HD-6801 manufactured by Hitachi Co., Ltd.
The configuration of information packets to be transmitted in the above-described network system, i.e., the data of a single frame comprises, as shown in FIG. 3, a marker slot SYN for identifying the frame, two channel slots SLT 1 and SLT 2 and subsequent break-down information slot SLT 3 for reporting the break-down state of the data transmission/reception network. The slot SLT 3 comprises an eight-bit code conveying the above-described break-down information.
For example, in the case when a break-down occurs in the data transmission/reception leg L.sub.3 between the stations C and A, the station A detects the break-down in the data transmission/reception leg L.sub.3 after it fails to receive any data for a period exceeding a predetermined time. Thereafter, the station A transmits a byte to the station B in the next stage in which a most significant bit (MSB) D.sub.7 is set to a "1" and the remaining bits are set to "0" as represented by (10000000) in binary form and by "80.sub.H " in hexadecimal-coded decimal form (H denotes a hexadecimal-coded decimal notation).
The station B determines from the above-described byte received from the station A in which the most significant bit is "1" that there is a break-down in the data transmission/reception route and furthermore sends the same information on the break-down to the station C. Together with the transmission of the break-down information to the station C, the direction of the transmission/reception direction is switched at predetermined intervals after the passage of a predetermined time T.sub.B.
In a similar way, the station C also recognizes that a break-down has occurred from the break-down-representative data received from the station B and thereafter switches its direction of communication repeatedly at predetermined intervals after the passage of a predetermined time T.sub.C.
The above-described operation is diagrammatically shown in FIG. 4. In this drawing, prior to the occurrence of the break-down (prior to detection of the break-down at time t.sub.0), the communication direction of all of the stations A, B, and C is constant and in the counterclockwise direction (represented by L in this drawing). However, after the time t.sub.0, each station performs bidirectional communication by switching the transmission directions between the counterclockwise (L) and clockwise (R) directions at a constant frequency.
In this way, after the break-down of the data transmission/reception route is detected, all of the information can be transmitted to all of the stations if each station is transferred into an operation mode in which the direction of the data transmission/reception is repeatedly switched at a constant frequency.
However, as described above, if the period of reversal of the communication direction during bidirectional communication is set to the same period T.sub.1 for all of the stations, each station will not be able to recognize whether the information is received by the destination station. Therefore, in order to ascertain that the information is received by the destination station, it is necessary to increase the number of times the data transmission is repeated or to prolong the reversal period of the above-described transmission direction, thus reducing the transmission speed accordingly.
In addition, since the period of reversal of the above-described communication direction is carried out by means of independent timers incorporated in each station, there may be some deviation of reversal timing of the communication direction among the stations because of the tolerances in accuracy of the respective timers incorporated in the stations. Therefore, the deviation will increase as time passes even though the deviation will be small initially. Consequently, such a condition will result in the communication directions of adjacent stations being completely reversed (for example, in the case when a specific station is transmitting in the counterclockwise direction, the adjacent station performs communication in the clockwise direction). In this case, neither transmission nor reception of data between adjacent stations is possible.